Condensation on the engine oil cap

From the description the issue is most likely just condensation from the air collecting in the engine because of the distance you drive. The engine and oil never get up to a high enough temperature to evaporate the moisture that collects from the temperature changes. I would suggest wiping the goo off, then take it for a long drive, you want it to get up to full operating temperature and stay there a while. Then once you get back, change the oil and filter to remove anything left behind. The problem with moisture in the oil and engine is that it can become acidic and form rust and sludge. To counter that you have a couple options, take it on a longer drive once a week or so , or change the oil more often to remove the contaminants. Either method works.

This is a general guide to changing your oil:

https://www.2carpros.com/articles/how-to-change-engine-oil-and-filter

Now if you run it a while and then check the oil and discover it looks like a chocolate milkshake, then I would worry about a head gasket or internal leakage. However this doesn't sound like that.